1. Field of the Invention
The present invention relates to hybrid electro-optical converter modules, and more particularly, to modules adapted for use in coupling information between an optical fiber and electrical wires and vice versa.
2. Description of the Prior Art
Optical fibers are rapidly becoming the transmission means of choice for conveying information between two different locations. The advantages of using optical fiber transmission lines are well known, in that they provide significantly greater bandwidths and information-carrying capacity. In most instances, optical information being transmitted over an optical fiber, either in a single direction or bidirectionally, must undergo an electro-optical conversion at each terminal of an optical fiber link, since the information is still usually processed in its electrical form. Electro-optical converting systems have been designed for both single and bidirectional transmission of optical information, and these systems have typically been constructed using discrete components to which the optical fibers are spliced. It is well recognized that it is a demanding task to connect small optical fibers to the individual components of a converting system.
Attempts have been made to integrate these components into a single unit generally referred to as a module. The modules usually require active devices in the form of optical detectors and optical sources and a passive component in the form of an optical coupler or optical splitter, with optical fibers used within the module to connect the active elements to the passive coupler. Connecting the optical fibers to these active and passive devices was a difficult task and, in many cases, required the use of special lenses to focus the light onto the end surfaces of the fiber and the appropriate areas on the active and passive devices. The need for the lenses and, particularly, the passive coupler device, resulted in the modules being rather large and bulky, with marginal performance capability due to electrical cross-talk between the active devices and poor isolation between transmitted and received optical information.
In many instances, it is desirable to provide for the bidirectional transmission of optical information; in such cases, the optical information is transmitted at different wavelengths in each direction, necessitating the use of a wavelength selective coupler which, in many cases, was large and did not provide satisfactory wavelength separation between the transmitted and received optical information.
U.S. Pat. No. 4,844,573 issued to Gillham et al discloses several embodiments of electro-optical converters provided in module form. Most of the converters of Gillham et al included three ports: one port for connection to an optical fiber link for bidirectional transmission, one port containing an electro-optical interface in the form of an optical detector, and the third port being an electro-optical interface in the form of an optical light source. The coupler utilized in Gillham et al is a fused fiber coupler. When fused fiber couplers are used the two active components are disposed on the same side of the module, so as to be in close proximity to each other. Such an arrangement can lead to increased electrical cross-talk between the active components unless special efforts are made to separate the components or electrically isolate them. Fused fiber couplers do not have the best wavelength isolation characteristics, and the transmitted and received signals are not adequately separated.
A preferred form of coupler, where wavelength selectivity is desired, is a coupler exhibiting dichroic properties. Such a coupler utilizes a multi-layer of dielectric coatings to establish an interface that is capable of reflecting a particular wavelength of light while transmitting a second wavelength. U.S. Pat. No. 4,296,995 issued to Bickel discloses an optical fiber beam splitter coupler employing coatings with dichroic properties. The device of the Bickel patent, however, provided only a coupling mechanism and, in addition, was rather large and bulky, since the fibers from which it was constructed were encapsulated in a body of potting compound.
Thus, the prior art did not provide a small, compact electro-optical module having low cross-talk and good wavelength separation.